This invention relates generally to steering columns and, more particularly, to motor vehicle steering columns which are separable in the event of impact.
When a motor vehicle is subjected to a front impact, the steering column and the steering wheel of the vehicle have a dangerous tendency to be displaced toward the driver. Various arrangements have been proposed to provide a separation of the steering column in order to avoid such displacement of the steering column and steering wheel.
One such proposed arrangement relies upon a frictional fit of several connecting pins to connect two shaft sections of the steering column. The connecting pins extend slidably from one shaft section through sleeves on the other shaft section. The sleeves are made of a high friction plastic to provide gripping of the connecting pins to ensure controlled sliding between the connecting pins and the sleeves. When an axial force is applied to the shaft sections of the steering column, the connecting pins slide completely from the sleeves to allow the two shaft sections to be released from each other.
Since a separable connecting device for a steering column is to be a safety feature, the device must meet extremely high reliability requirements to be useful. If the device separates at too low an axial force, the driver would lose steering control when the vehicle is not disabled by a severe impact. And, if the device separates at too high an axial force, the driver would receive no benefit from the device when the vehicle suffers a severe impact. For that reason, a "force window" may be defined between minimum and maximum axial forces within which separation of the steering column should occur.
Because the previously described proposed arrangement relies upon friction to provide the separation force, the actual force required to effect separation of the two shaft sections cannot be controlled accurately. The frictional force varies widely, for example, due to thermal expansion of the pin and due to thermal relaxation of the plastic sleeve. In addition, the performance of the plastic sleeve deteriorates over the life of the motor vehicle. And, tolerances must be closely controlled to produce the necessary interference fit, resulting in considerable manufacturing difficulty and high cost.
Even if such a proposed separation device could provide separation over a specified axial force window over a wide temperature range, there is no assurance that the intended function will be performed in the event of a severe vehicle impact. The environment of a vehicle crash is only imperfectly understood, resulting in uncertainty as to the direction of load on the steering column and as to the amount of the axial component of the load.
The foregoing illustrates limitations known to exist in present separable connecting devices for steering columns. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one of more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.